Caption: Interior of the converted S-IVB rocket stage that formed the largest part of Skylab. Astronauts lived inside the S-IVB's liquid hydrogen tank and used the stage's smaller liquid oxygen tank as a trash dump. Image: NASA.

Talking to the Farside: Apollo S-IVB Stage Relay (1963)

Apollo-Saturn S-IVB stage. Image: NASA.

The S-IVB rocket stage played several important roles in NASA’s 1960s and 1970s manned space programs. The 58.4-foot-long, 21.7-foot-wide stage, which comprised a single restartable J-2 rocket engine, a forward liquid hydrogen tank, and an aft liquid oxygen tank, served as the second stage of the two-stage Apollo Saturn IB rocket and the third stage of the three-stage Apollo Saturn V.

Cutaway of S-IVB stage configured for use as Saturn V third stage. Image: NASA.

The Saturn IB S-IVB’s J-2 engine would ignite at an altitude of about 42 miles and burn until it placed a roughly 23-ton payload into low-Earth orbit. After that, it would shut down and the spent stage would separate. The Saturn V S-IVB’s J-2, on the other hand, would ignite twice to accelerate the stage and its payload: once for 2.5 minutes at an altitude of about 109 miles and again for six minutes about two and a half hours later. The first burn would place the S-IVB and payload into a low parking orbit between 93 and 120 miles above the Earth; the second would place the S-IVB and payload onto a path that would intersect the moon, about 238,000 miles away, about three days after Earth launch. Departure for the moon was called Translunar Injection (TLI).

Cutaway of S-IVB stage configured for use as Apollo-Saturn IB second stage. Image: NASA.

During Apollo moon landing missions, the payload was a three-man Command and Service Module (CSM) and a Lunar Module (LM) moon lander. The astronauts would separate the CSM from the four-segment shroud linking it to the S-IVB about 40 minutes after TLI. They would then maneuver it clear of the S-IVB and turn it end for end so that its nose pointed back at the top of the stage. The shroud segments, meanwhile, would hinge back and separate to reveal the LM spacecraft mounted atop the S-IVB. The crew would guide the CSM to a docking with the LM; then, about 50 minutes after docking, the joined CSM and LM would move away from the S-IVB. The stage would then vent residual propellants and ignite auxiliary rocket motors to place itself on a course away from the CSM-LM combination.

Roughly 60 hours after launch from Earth, the docked CSM and LM would enter the moon’s gravitational sphere of influence. About 12 hours later, they would pass behind the moon over Farside, the lunar hemisphere turned always away from Earth. There, out of visual, radar, and radio contact with Earth, the CSM would ignite its Service Propulsion System (SPS) main engine to slow itself and the LM so that the moon’s gravity could capture them into lunar orbit. This critical maneuver was called Lunar Orbit Insertion (LOI). Orbital mechanics dictated that LOI should occur over the center of Farside.

Apollo 8 mission profile. The first manned mission to the moon, Apollo 8 included no Lunar Module. It did, however, pioneer the TLI, LOI, and TEI maneuvers for subsequent Apollo missions. The dashed line shows the path of the S-IVB stage after TLI and Apollo 8 spacecraft separation. Image: NASA.

A few hours later, two astronauts would separate from the CSM in the LM. They would fire the moon lander’s throttleable descent stage engine – again over Farside, as dictated by orbital mechanics – to begin their descent toward their pre-selected landing site on Nearside, the lunar hemisphere turned always toward Earth. Following a safe landing and a period of surface exploration (less than one Earth day for the earliest Apollo landing missions), the LM ascent stage would lift off. About two hours later – again over the moon’s hidden hemisphere – the CSM would rendezvous and dock with the LM. The lunar landing crew would rejoin the CSM pilot, the astronauts would cast off the LM ascent stage, and preparations would begin to ignite the SPS to depart lunar orbit for Earth. The critical lunar-orbit departure maneuver, also carried out over Farside, was called Trans-Earth Injection (TEI).

The S-IVB stage would, meanwhile, swing past the moon and enter orbit around the Sun. Although it would travel to the moon and beyond, as of early 1963 no one had identified any further role for the S-IVB after the CSM and LM cast it loose.

For six months in 1963, engineers at The Bissett-Berman Corporation in Santa Monica, California, working on contract to NASA Headquarters, studied another use for the Apollo-Saturn V S-IVB stage. In a series of “Apollo Notes” beginning in March of that year, they identified a need for a relay satellite to enable Earth-based radar tracking of the Apollo CSM and LM while they carried out crucial maneuvers over Farside. They then proposed that the spent S-IVB be outfitted to serve as that relay satellite.

The first note, authored by H. Epstein and based on a concept suggested by L. Lustick, proposed a radar relay satellite for tracking the Apollo CSM during LOI and CSM rendezvous and docking with the LM ascent stage. Epstein and Lustick’s satellite would include an omnidirectional antenna for near-lunar operations and, for “deeper phase operation,” a steerable four-foot parabolic dish antenna.

The relay satellite, Epstein wrote, would separate from the Apollo spacecraft before LOI, then would fly past the moon on a path that would keep both Earth and most of Farside in view during LOI and CSM-LM rendezvous and docking. The omni antenna would relay radar from Earth until the satellite was 40,000 kilometers from the moon, then the dish would take over.

The second Bissett-Berman Apollo Note, dated April 16, 1963, raised the possibility of placing a “special purpose relay package” on the S-IVB stage. The package would either remain attached to the stage or would eject from it when activated. The Apollo Note’s author, L. Lustick, attributed the S-IVB relay concept to one Dr. Yarymovych, whose affiliation was not stated.

For his analysis, Lustick assumed that the S-IVB would retain enough propellants for its J-2 engine to restart a third time shortly after CSM-LM separation, raising its speed by 160 feet per second. He calculated that, at the time of LOI, the S-IVB or relay package would have in view simultaneously both Earth and more than three-quarters of Farside. At the time of CSM docking with the LM ascent stage, about 100 hours after Earth launch, the relay would have in view Earth and a little more than two-thirds of Farside. Throughout the approximately 28-hour period between LOI and CSM rendezvous with the LM ascent stage, the S-IVB would remain within 143,000 miles of the moon.

The ring-shaped Instrument Unit, a rocket guidance system, was mounted on top of the S-IVB stage in both the Saturn V and Saturn IB rockets. Image: NASA.

The S-IVB would rely for attitude control on the ring-shaped Instrument Unit (IU), the Saturn V’s “electronic brain.” The IU, located at the front of the S-IVB, was not intended to operate for more than a few hours, so would need modifications to ensure that it could reliably stabilize the S-IVB throughout the relay period. In an addendum to Lustick’s Apollo Note dated April 18, 1963, H. Epstein looked at simplifying the S-IVB Farside Relay concept by assuming that the S-IVB would lack attitude control while it acted as a data relay.

Replacing steerable dish antennas – one for Earth-S-IVB communication and one for S-IVB-Apollo CSM communication – with two passive omnidirectional antennas would permit data relay no matter how the spent S-IVB became oriented, Epstein wrote. The use of relatively low-power omni antennas would produce few problems as far as Earth-S-IVB communication was concerned, for NASA could call into play larger antennas on Earth to ensure reception of the weakened signal. Epstein proposed increasing from four feet to five feet the planned diameter of the dish antenna on the CSM to enable it to receive data from Earth relayed through the S-IVB-CSM omni antenna. He noted, however, that, even with a larger CSM dish antenna, radio interference from the Sun might stymie the omni antenna relay concept.

An undated Apollo Note by Lustick and C. Siska explored the S-IVB Farside Relay concept in yet greater detail, and included evidence of NASA interest in the scheme: for the first time, the authors cited constraints imposed by NASA Headquarters, which managed the Bessitt-Berman contract. The space agency told Bissett-Berman to assume that the S-IVB could increase its speed by up to 1000 feet per second for up to about seven hours after TLI, and that the maximum range between the S-IVB Farside Relay and the CSM should not exceed 40,000 nautical miles throughout the relay period.

NASA, Lustick and Ciska explained, sought to learn whether relay of voice (not only data or radar) would be possible using an S-IVB Farside Relay during the roughly 30-hour period between LOI (a “particularly important” time to have voice relay capability, NASA asserted) and CSM-LM ascent stage rendezvous and docking. The authors found that boosting the S-IVB’s speed by 1000 feet per second 7.6 hours after TLI would place it on a path to relay voice between Earth and Farside from 72 hours after Earth launch until 102 hours after launch, at which time the S-IVB would reach NASA’s 40,000-nautical-mile limit. In fact, they found that the S-IVB would have Farside in view as early as 60 hours after Earth launch, though this was of purely academic interest, since no spacecraft would be over the moon’s hidden hemisphere at that time.

Lustick and Ciska also noted that the S-IVB would pass out of sight behind the moon (that is, become occulted by the moon) as viewed from Earth 102 hours after Earth launch. They added, however, that slight adjustments in S-IVB boost direction would postpone loss of Earth contact with the S-IVB Farside Relay for long enough to ensure that voice communication could continue during CSM rendezvous with the LM ascent stage.

In Bissett-Berman’s penultimate examination of the S-IVB Farside Relay concept, author Ciska noted that a 1000-foot-per-second boost could occur as early as TLI. This would, however, leave no propellant margin for later correction of S-IVB boost aim errors. On the other hand, S-IVB attitude control was expected to “drift” over time, making accurate boost pointing later than TLI increasingly unlikely. Furthermore, boil-off of liquid hydrogen from the S-IVB stage would rapidly reduce the amount available to fuel a later boost. Both of these factors gave weight to the concept of an “all-or-nothing” early boost.

Ciska noted also that, regardless of the S-IVB boost aim point selected, the stage would pass out of sight behind the moon as viewed from Earth for roughly half an hour at some point along its curved path during the voice relay period. For a 1000-foot-per-second boost applied 7.6 hours after TLI with an aim point slanted 100° relative to a line linking the Earth and moon, for example, the half-hour occultation would occur about 99 hours after Earth launch.

The last Bissett-Berman Apollo Note devoted to the S-IVB Farside Relay concept, also by Ciska and dated August 20, 1963, was an extension of his earlier note. In it, he examined an S-IVB boost 4.15 hours after TLI and additional effects of boost direction. Ciska did not attempt to plot S-IVB attitude drift or liquid hydrogen boiloff rates; nevertheless, he proposed as realistic a 700-foot-per-second boost 4.15 hours after TLI with an aim point slanted 100° relative to the Earth-moon line. Following this maneuver, the S-IVB Farside Relay would pass out of view of Earth for about 30 minutes a little more than 83 hours after Earth launch and would pass beyond NASA’s 40,000-nautical-mile limit about 103 hours after launch.

Though the Bissett-Berman scheme was not taken up, S-IVB stages did play key non-propulsive roles in NASA’s manned space program. NASA converted Saturn IB S-IVB 212 into the Skylab 1 Orbital Workshop. Skylab was launched into low-Earth orbit on the last Saturn V to fly and staffed by three three-man crews in 1973-1974. Saturn V S-IVB 515, originally intended to boost the Apollo 20 mission to the moon, was converted into the Skylab B workshop, but was not launched and wound up on display in the National Air and Space Museum in Washington, DC.

Interior of the converted S-IVB rocket stage that formed the largest part of Skylab. Astronauts lived inside the S-IVB's liquid hydrogen tank and used the stage's smaller liquid oxygen tank as a trash dump. Image: NASA.

Of the 10 Apollo Saturn V S-IVBs that departed low-Earth orbit between 1968 and 1972, half reached orbit about the Sun and half were intentionally crashed into the moon. The Apollo 8, 9, 10, 11, and 12 S-IVBs departed the Earth-moon system, while those that boosted Apollo 13, 14, 15, 16, and 17 out of low-Earth orbit toward the moon were intentionally impacted on the moon’s Nearside. The impacts were part of a science experiment: the seismic waves their impacts generated registered for hours on seismometers left behind on the lunar surface by earlier Apollo crews, helping to reveal to scientists the structure of the moon’s deep interior. In early 2010, NASA’s automated Lunar Reconnaissance Orbiter spacecraft imaged the crater left by the Apollo 13 S-IVB impact.

The Apollo 12 S-IVB, launched on November 14, 1969, flew past the moon too fast to receive a gravity-assist boost into orbit about the Sun, so circled the Earth in a loosely bound distant orbit until 1971, when, through gravitational perturbations from Earth, Sun, and moon, it finally escaped into solar orbit. It orbited the Earth again for about a year in 2002-2003, during which time it was observed and mistakenly identified for a time as a near-Earth asteroid.

The Apollo 13 S-IVB stage was the first of five intentionally crashed into the lunar surface as part of a seismic experiment. NASA's Lunar Reconnaissance Orbiter spacecraft imaged the fresh crater it left in the lunar surface. Image: NASA.